The invention relates to physical vapor deposition equipment such as are used in the fabrication of integrated circuits and semiconductor devices.
Physical vapor deposition (PVD) is a process by which a target material (e.g. Ti) is deposited onto an object (e.g. a semiconductor wafer) by means of a plasma. The process takes place in a vacuum chamber that contains an inert gas (e.g. argon). The plasma, which is generated in the chamber between the negatively biased target and the wafer, ionizes the inert gas. The positively charged ionized gas atoms are pulled toward the negatively biased target and impact it with sufficient energy to expel (i.e., sputter) atoms of target material from the target. The sputtered atoms from the target are propelled toward the wafer where they form a layer of deposited material.
The trajectories of the material coming off from the target during sputtering are distributed over a range of directions. Though typically most of the sputtered material travels in a direction that is normal to the target, a significant amount travels in other directions that diverge from the normal direction. The sputtered material which travels along the divergent directions tends to limit the definition that is obtainable at discontinuities on the wafer surface. More specifically, the material which travels along the non-normal trajectories deposits on the sidewalls of features such as thru-holes and vias, thereby limiting how small one can make those features. For holes that are too small, the material deposited on the sidewalls eventually closes up the hole and prevents any further material from being deposited at the bottom of the hole.
Collimation filters are used to filter out all sputtered material having a trajectory that diverges from the normal direction by more than a preselected angle. The collimation filter is placed between the target and the wafer. In general, it is a metal plate having a particular thickness with an array of holes passing through it. To maximize throughput, a honeycomb structure (i.e., a pattern of hexagonal holes) is used. The holes have a specified aspect ratio, i.e., the ratio of their length to their diameter. The aspect ratio determines the degree of filtering which takes place. A higher aspect ratio produces a narrower angular filter (i.e., the preselected angle is smaller). A consequence of using a higher aspect ratio, however, is a significantly reduced throughput. Thus, picking the appropriate thickness and hole size for the collimation filter is simply a question of process optimization.
Nevertheless, for a typical filter design in a conventional system, only about 20% of the material will make it through the filter. The rest of the material, which represents sputtered material that has a trajectory that diverges from the normal direction by more than the preselected angle, deposits on the filter. In this example, the collimation filter reduces throughput by a factor of 5.
To compensate for the reduced throughput, users typically increase the operational power on the target. For example, rather than running at a 5 kW power level, the power is increased to about 20 kW. The increased power levels increase the rate of sputtering. They also, however, introduce other problems, e.g. increased temperature of the wafer and unwanted material interactions in the deposited layer.